Recently, it has been required to measure a humidity of intake air in order to improve fuel consumption of an internal combustion engine and to clean exhaust gas.
The following humidity sensor has been known as one example of a humidity sensor measuring a humidity of intake air.
The humidity sensor includes a humidity detector, a temperature detector and a heater. The humidity detector has a humidity-sensitive film and detects a relative humidity of intake air based on a capacitance of the humidity-sensitive film. The intake air is taken into an internal combustion engine for a vehicle. The humidity-sensitive film is located in an intake passage where the intake air flows. The temperature detector detects a temperature of the humidity-sensitive film. The heater heats the humidity-sensitive film. The heater constantly heats the humidity-sensitive film to restrict dew condensation or fouling of the humidity-sensitive film. As such, humidity detection accuracy is maintained (for example, see JP 2015-87196 A).
For a factor of reducing the humidity detection accuracy, malfunction of the humidity detector itself is considered as well as the dew condensation or the fouling of the humidity-sensitive film.
However, in the humidity sensor disclosed in JP 2015-87196 A, the malfunction of the humidity sensor itself is not detected.
In the humidity sensor disclosed in JP 2015-87196 A, the heater continues to heat the humidity detector and the temperature detector. As a result, the temperature of the temperature detector is increased and detection accuracy of a temperature of the intake air is reduced. Also, the temperature of the humidity-sensitive film is increased as the temperature of the humidity detector is increased. As a result, detection accuracy of an absolute humidity is reduced. Furthermore, since the heater continues to heat, power consumption is also increased.